Stanford team tries for zippier Wi-Fi in crowded buildings

Having lots of Wi-Fi networks packed into a condominium or apartment building can hurt everyone's wireless performance, but Stanford University researchers say they've found a way to turn crowding into an advantage.

In a dorm on the Stanford campus, they're building a single, dense Wi-Fi infrastructure that each resident can use and manage like their own private network. That means the shared system, called BeHop, can be centrally managed for maximum performance and efficiency while users still assign their own SSIDs (service set identifiers), passwords and other settings, according to Yiannis Yiakoumis, a Stanford doctoral student who presented a paper at the Open Networking Summit this week.

There are Wi-Fi networks today, such as systems from Ruckus Wireless, that can be deployed across multi-unit buildings with some private control by individual residents. But the Stanford project is making this happen with inexpensive, consumer-grade access points and SDN (software-defined networking), on the foundation of open-source software.

In multi-unit housing, each household typically installs its own Wi-Fi network with a wired broadband link out to the Internet. Each of those networks may be powerful enough to give good performance under optimal circumstances within the owner's unit, but it may suffer from interference with all the other privately run networks next door.

Borrowing techniques from enterprise Wi-Fi, Yiakoumis and his colleagues built a shared network of APs (access points), in this case home units provided by NetGear. They modified the firmware of those APs, and using SDN, they virtualized the private aspects of the network experience.

In the Stanford researcher's model, residents can name and secure their own virtual networks as if they had bought and plugged in a router in their own rooms. They can also assign policies such as parental controls and prioritize their favorite applications for access to bandwidth. Then, wherever they go in the building, they can log into that same virtual network, Yiakoumis said.

Meanwhile, the underlying tasks of assigning client devices to particular channels and access points are centrally controlled to make the best use of the infrastructure. Where separately owned and managed APs may make poor use of the unlicensed frequencies available in the building, the centrally controlled network can use its universal view to arrange the resources most efficiently.

SDN places control of networks in overarching software rather than in the specialized network components that forward packets. BeHop uses software components including the OpenWRT Linux distribution for Wi-Fi routers and the Open VSwitch virtual switch, which is included in the Linux kernel. While most of the software used in the project is open source, the team has developed some code it hasn't had time to release as open source, Yiakoumis said. It plans to do so later.

BeHop also differs from enterprise wireless LANs, and from residential systems based on enterprise-class APs such as Ruckus', with its consumer-grade access points. The Stanford team used the approach of blanketing the dorm with inexpensive APs and skipping the typically expensive and time-consuming task of conducting a site survey for optimal placement. They don't yet have performance numbers for the network, but they expect to produce those in the coming months.

Ruckus says its enterprise-class APs, which cost anywhere from US$500 to $1,000 each, are built to use spectrum better than consumer-grade units priced at $200 and below. The Ruckus APs point their signals at a user's device rather than blasting transmissions across a wide area, which helps no matter how the network is managed, said David Callisch, vice president of corporate marketing.

Wi-Fi routers that consumers buy for their own units don't clash with each other very often, because they usually don't transmit on the same channel at the exact same time, said Farpoint Research analyst Craig Mathias. As more devices come out with radios for both the crowded 2.4GHz band and the more spacious 5GHz band, they'll have even more channels to choose from. But demands on all Wi-Fi frequencies will continue to grow, he said.

"It hasn't been as big a problem as people are making it out to be," Mathias said. "Over time, though, it will become more of a problem."

Enterprise Wi-Fi systems have sophisticated mechanisms for dividing up spectrum to provide the most possible capacity, but consumer-grade routers have very little. At most, a router that's set to automatically pick a channel will check to see which one's already busy, but it may not do that quickly or often enough, Mathias said.

"If everybody uses their own router, you don't have a prayer" of getting optimal spectrum use, Mathias said. That said, when performance lags, it's usually because the shared wired connection to the Internet is too narrow, he said. Farpoint recommends multi-unit dwellings use centrally deployed and managed Wi-Fi with enterprise-class access points.

The technology being developed at Stanford could be offered by access-point vendors, a managed service provider, a building owner, or an Internet service provider, Yiakoumis said. He and his colleagues are leaving the business model to others. It would work best if the residents shared the same broadband service, he said. Because Wi-Fi uses unlicensed spectrum, other residents might set up their own Wi-Fi routers anyway. But the more who participated, the better the network's overall performance, he said.

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